Both large and small storage tanks have been used for containing a variety of liquids. Some of these liquids, such as water, if leaked from a storage tank would have no detrimental effect on the environment, while others, such as hydrocarbon based fuels, could have a very harmful effect.
Concerns regarding the potentially hazardous effects certain liquids may have on the environment and on people exposed to the liquids have made the early detection of leaks from storage tanks increasingly important. In addition, the early and accurate detection of storage tank leaks allows the loss of valuable liquids to be minimized.
In many cases, leaks in storage tanks are not visually identifiable. For example, there could be a leak from the bottom of the tank. Moreover, liquids leaking from storage tanks are quickly absorbed into the ground, thereby providing little visual evidence of the leak. Therefore, only very large leaks are detectable by visual inspection of the area surrounding the tanks or the tanks themselves.
Visual detection of leaks by inspection of the change in the liquid level of the storage tank is also extremely difficult. This is particularly true in the case of large diameter storage tanks. A problem associated with large diameter storage tanks is that even small leaks, which are visually imperceptible, translate into large volume losses of the tank liquid. As such, before a leak is visually detectable, significant liquid loss and/or significant environmental damage may be experienced.
Additional problems associated with visually inspecting the liquid level in tanks to determine if there is leakage is that inaccuracies are introduced by physical factors that affect the tank liquid. For example, the volume may change due to temperature variations or the motion of the liquid due to the wind.
In the past, a method of detecting leaks in storage tanks has involved filling a tank with water, or other liquid, to a high level to subject the tank bottom to as great a pressure as possible. A sight glass was provided to permit visual inspection the level of water or liquid in the tank.
Another method has been to add water to tanks containing a stored liquid in order to limit the harmful effects of a leak in the tank. Since water has a greater specific density than the liquids usually stored in the storage tanks and is immiscible with most of these liquids, the water settles to the bottom of the tank and an interface is formed between the water and the stored liquid. Thus, in the case of a bottom leak, no tank liquid would leak from the tank until all of the water had leaked out. A sight glass may be used to inspect the level of the interface between the water and the stored liquid.
Detection of tank leaks by using either of these methods is limited by sight glass meniscus resolution. For example, if the sight glass meniscus resolution is limited to approximately a 1/16 inch variation in liquid level, there can be significant liquid losses before leakage is detected, as shown in Table 1.
TABLE 1 ______________________________________ VERTICAL CYLINDRICAL TANKS TANK DIA- VOL/FT. DEPTH VOL/1/16" DEPTH METER (FEET) (GALLONS) (GALLONS) ______________________________________ 10 587.6 3.06 25 3672.2 19.125 50 14688.9 76.505 75 33050.0 172.135 100 58755.6 306.02 125 91805.7 478.155 150 132200.2 688.54 175 179939.1 937.18 200 235022.5 1224.075 ______________________________________
Moreover, most sight glasses, even when filled with liquid, are of small mass, and the level in sight glasses is greatly affected by the wind. The level of the liquid in the sight glass also is significantly affected by temperature changes.
In the past, leaks have also been detected using a differential pressure transducer that connects to the liquid volume inside the tank through a pipe extending from the base of the tank. One side of the pressure transducer is connected to the storage tank through a pipe and a valve, and the other side of the pressure transducer is connected to an external standpipe. The transducer compares the liquid level in the tank to the level of liquid in the standpipe. Leaks, therefore, are detected by changes in the differential pressure at the pressure transducer.
The differential pressure transducer method reduces some of the inaccuracies associated with the thermal expansion and contraction of the liquid due to temperature. However, the thermal expansion and contraction of the piping that connects this system to the tank, standpipe, and within the standpipe itself is significant. This may cause pressure variations of the same magnitude as would be caused by small leaks. Thus, this method is ineffective to detect such small leaks. In addition, this method is practical only for above-ground tanks because the cost of modifying underground or partly in-ground tanks is prohibitive, and because most underground tanks are not of an uniform cross-sectional area at different elevations in the tank.
Even the installation of such systems on above-ground tanks requires substantial modifications to the tank. For example, the tank may need to be emptied, purged of hydrocarbon vapors, and holes must be put in the tank in order to connect the standpipe and differential transducer to the tank.
In the alternative, the tanks may be hot tapped. This requires the welding of a pipe to the outside wall of the tank and then drilling a hole through the tank wall. There is the risk that the heat generated during this operation may ignite the liquid stored in the tank or that this operation will structurally damage the tank itself.
In addition, these types of systems are subject to inaccuracies caused by any floating roof which is in contact with the stored liquid. This is due to random pressure changes which may result from the movement of the roof.
Accordingly, there is a need for a leak detection system for rapidly detecting leaks in storage tanks that does not suffer from the problems associated with prior systems.